Showing papers on "Micromechanics published in 1984"

Simplified composite micromechanics equations for strength, fracture toughness and environmental effects

Abstract: A unified set of composite micromechanics equations of simple form is summarized and described. This unified set includes composite micromechanics equations for predicting: (1) ply in-plane uniaxial strengths; (2) through-the-thickness strength (interlaminar and flexural); (3) in-plane fracture toughness; (4) in-plane impact resistance; and (5) through-the-thickness (interlaminar and flexural) impact resistance. Equations are also included for predicting the hygrothermal effects on strength, fracture toughness and impact resistance. Several numerical examples are worked out to illustrate the ease of use of the various composite micromechanics equations.

Simplified composite micromechanics equations of hygral, thermal, and mechanical properties

Abstract: A unified set of composite micromechanics equations of simple form is summarized and described. This unified set can be used to predict unidirectional composite (ply) geometric, mechanical, thermal and hygral properties using constituent material (fiber/matrix) properties. This unified set also includes approximate equations for predicting (1) moisture absorption; (2) glass transition temperature of wet resins; and (3) hygrothermal degradation effects. Several numerical examples are worked-out to illustrate ease of use and versatility of these equations. These numerical examples also demonstrate the interrelationship of the various factors (geometric to environmental) and help provide insight into composite behavior at the micromechanistic level.

Simplified Composite Micromechanics Equations for Strength, Fracture Toughness, Impact Resistance and Environmental Effects.

Abstract: : A unified set of composite micromechanics equations of simple form is summarized and described This unified set includes composite micromechanics equations for predicting (1) ply in-plane uniaxial strengths; (2) through-the-thickness strength (interlaminar and flexural); (3) in-plane fracture toughness; (4) in-plane impact resistance; and (5) through-the-thickness (interlaminar and flexural) impact resistance Equations are also included for predicting the hygrothermal effects on strength, fracture toughness and impact resistance Several numerical examples are worked out to illustrate the ease of use of the various composite micromechanics equations The numerical examples were selected, in part, to demonstrate the interrelationships of the various constituent properties in composite strength and strength-related behavior, to make comparisons with available experimental data and to provide insight into composite strength behavior

Synthesis and toughness properties of resins and composites

Abstract: Tensile and shear moduli of four ACEE (Aircraft Energy Efficiency Program) resins are presented along with ACEE composite material modulus predictions based on micromechanics. Compressive strength and fracture toughness of the resins and composites were discussed. In addition, several resin synthesis techniques are reviewed.

Investigation of the Relations Between Neat Resin and Advanced Composite Mechanical Properties . Volume 1. Results.

Abstract: A detailed evaluation of one untoughened epoxy baseline resin and three toughened epoxy resin systems was performed. The Hercules 3502, 2220-1, and 2220-3, and Ciba-Geigy Fibredux 914 resin systems were supplied in the uncured state by NASA-Langley and cast into thin flat specimens and round dogbone specimens. Tensile and torsional shear measurements were performed at three temperatures and two moisture conditions. Coefficients of thermal expansion and moisture expansion were also measured. Extensive scanning electron microscopic examination of fracture surfaces was performed, to permit the correlation of observed failure modes with the environmental conditions under which the various specimens were tested. A micromechanics analysis was used to predict the unidirectional composite response under the various test conditions, using the neat resin experimental results as the required input data. Mechanical and physical test results, the scanning electron microscope observations, and the analytical predictions were then correlated.

Characterization of Interlaminar Crack Growth in Composites with the Double Cantilever Beam Specimen

Abstract: A project to examine the double cantilever beam specimen as a quantitative test method to assess the resistance of various composite materials to interlaminar crack growth is discussed. A second objective is to investigate the micromechanics of failure for composites with tough matrix resins from certain generic types of polymeric systems: brittle thermosets, toughened thermosets, and thermoplastics. Emphasis is given to a discussion of preliminary results in two areas: the effects of temperature and loading rate for woven composites, and the effects of matrix toughening in woven and unidirectional composites.

Application of finite element substructuring to composite micromechanics

Abstract: Finite element substructuring is used to predict unidirectional fiber composite hygral (moisture), thermal, and mechanical properties. COSMIC NASTRAN and MSC/NASTRAN are used to perform the finite element analysis. The results obtained from the finite element model are compared with those obtained from the simplified composite micromechanics equations. A unidirectional composite structure made of boron/HM-epoxy, S-glass/IMHS-epoxy and AS/IMHS-epoxy are studied. The finite element analysis is performed using three dimensional isoparametric brick elements and two distinct models. The first model consists of a single cell (one fiber surrounded by matrix) to form a square. The second model uses the single cell and substructuring to form a nine cell square array. To compare computer time and results with the nine cell superelement model, another nine cell model is constructed using conventional mesh generation techniques. An independent computer program consisting of the simplified micromechanics equation is developed to predict the hygral, thermal, and mechanical properties for this comparison. The results indicate that advanced techniques can be used advantageously for fiber composite micromechanics.

Micromechanics analysis of unidirectional fibrous reinforced composite materials

Abstract: Composite materials are heterogeneous materials consisting of two or more discrete components. The primary role of micromechanics analysis is to define the constitutive mechanical behaviour of these materials so that they can be subsequently treated as macroscopically homogeneous materials in studies based on macro mechanics. A complementary objective is the examination of the microscopic mechanical interaction between the constituent phases of the heterogeneous material. Three dimensional finite element analysis is utilised for this work. The bulk constituents and fibre matrix interface are represented by isoparametric elements with quadratic displacement fields. Non linear behaviour is considered in terms of an elasto plastic model based on the von mises yield criterion, for the solid constituents and of a debonding model, allowing delamination and tangential slippage, for the interface (a).

Investigation of Toughened Neat Resins and Their Relations to Advanced Composite Mechanical Properties

Abstract: Four epoxies (Hercules 3502, Hercules 2220-1 and 2220-3, and Ciba-Geigy Fibredux 914 epoxy) were evaluated to judge their merits within the ACEE program and also to provide material property input for a micromechanics finite element program. A casting procedure was developed for the four matrix systems mentioned and mechanical properties were studied at three test temperatures and two moisture contents. A scanning electron microscope study was performed to catalogue fracture surfaces for the unreinforced (neat) epoxies. In addition, tension testing, shear testing, a limited amount of fracture toughness testing, and thermal expansion coefficient and moisture expansion coefficient testing were carried out. Mechanical material properties were generated for input into a micromechanics program and to characterize these four matrix systems.

Sensitivity Study of Modified Micromechanics Relations for Composite Materials

Abstract: Simple modified rule of mixture relations to determine the elastic properties of a composite ply are taken from the literature and studied. Fiber properties are back calculated for AS1 fibers. Plots are given to emphasize the sensitivity of the ply proper ties to perturbations in each of the constituent material properties. Particular attention is paid to ply transverse modulus.

Materials aspects of crack tip yielding and subcritical crack growth in engineering alloys

Abstract: The micromechanics of crack opening, and plastic yielding within the highly deformed near-tip region, are described. Recent high resolution experimental observations are compared with the results of continuum analyses, and deviations due to material and environmental influences noted. The role of detailed crack yielding information in the modeling and understanding of subcritical crack advance is discussed.

Micromechanics of fracture in microalloyed steel

Abstract: A quantitative investigation of the micromechanics of fracture has been carried out in hot-rolled microalloyed steel. The inclusion size and inter-inclusion spacing distribution have been determined using statistical analysis. Tensile properties and toughness data have been obtained and used for determination of stress and strain distribution in the vicinity of crack tip. The fracture strain and dimple sizes for fracture are found to be highly sensitive to triaxiality factor. The process zone size has been determined from the critical strain level in the fracture toughness (FT) specimen and related to the dimple size. The effective inclusion size governing the ductile fracture process has been identified.

In situ delamination of graphite/epoxy composite materials

Abstract: In situ delamination of two graphite/epoxy composites has been conducted in the scanning electron microscope to study the effect of rubber particle toughening of the matrix on the micromechanics of fracture. Extensive tearing and microcracking was observed during the fracture. The fractographs indicate that the elastomer contributes significantly to the toughening of the composite by initiating localized deformation and/or microcracking in the vicinity of the particles. A study of coated versus uncoated specimens have proved conclusively that the coating enhances picture quality without introducing any artifacts into the micrographs. The entire delamination sequence has been videotaped off the SEM to provide a dynamic record of the actual fracture process.

Processing and Properties of Airframe Materials.

Abstract: : This annual report describes progress during the first year of the three year research program to study the relationship between microstructure and processing conditions, and the effect of processing conditions on the performance of structural airframe materials. Part I of this program is examining the influence of beta processing methods on the interaction of fatigue cracks with the microstructural elements. three beta phase processing methods have been chosen to provide variations in beta phase continuity and alpha phase plate size. The microstructure of the beta processed materials has been quantitatively characterized using SEM, TEM and STEM. Fatigue crack propagation results are also presented for each of the beta annealing treatments. In Part II, the micromechanics of superplastic deformation behavior. Experiments have been performed with 7475 Al having various grain sizes. The flow stress vs strain rate behavior for the mixed grain size materials is best described using the iso-strain rate concept. Observations of dynamic grain growth and dynamic recrystallization have led to new perceptions of how these processes may alter the mechanical response of the materials during superplastic deformation. Based on these observations, the new model of the superplastic deformation process is outlined. (Author)

Crack propagation studies of a rubber-toughened epoxy resin in the scanning electron microscope

Abstract: The crack tip micromechanics in a commercial, rubber toughened epoxy resin have been studied by conducting crack propagation experiments in a scanning electron microscope. The method gives a clear picture of crack tip deformation and crack tip opening displacement. The critical crack shear opening displacement provides a satisfactory estimate of the critical strain energy release rate GIC of this material. Also, the material exhibits a maximum shear strain of about 47% at the crack tip.